1. Field of the Invention
The present invention relates to electrical and electronic circuits and systems. More specifically, the present invention relates to power amplifiers for communications systems and predistortion linearizers used in connection therewith.
2. Description of the Related Art
Power amplifiers are used in a variety of communications applications. Power amplifiers not only typically exhibit non-linear distortion but also possess memory effects. While non-linear distortion follows the power amplifier characteristics, the memory effects depend on signal characteristics (e.g., signal bandwidth and also transmit power level.)
Conventional techniques use Lookup Table (LUT) based methods to generate inverse transfer functions in both amplitude and phase to correct for non-linearity in the output of power amplifiers. However these techniques do not effectively handle memory effects and thus provide very moderate linearization improvements.
More importantly, power amplifiers are generally expensive and consume much power. Multiple carrier applications, such as cellular telephone base stations, are particularly problematic inasmuch as a single amplifier is typically used with each carrier signal. Conventional approaches combine separate power amplifiers to transmit multiple carrier signals. However, this approach is also expensive and power intensive.
Further, for maximum efficiency, each power amplifier must be driven close to its saturation point. However, as the power level is increased, intermodulation distortion (IMD) levels increase. Hence, the output power level must be ‘backed-off’ to maintain acceptable ACPR (adjacent channel power ratio) levels. Unfortunately, the required power back-off to meet government (e.g. FCC) specified IMD levels limits the efficiency of the power amplifier to relatively low levels and does not offer a solution that is sufficiently cost effective for certain current requirements.
An alternative approach is to use a predistortion linearizer. This technique allows power amplifiers to operate with better power efficiency, while at the same time maintaining acceptable IMD levels. However, conventional digital predistortion linearizers have, been shown to reduce the IMD levels by only about 10-13 dB for signal bandwidths in excess of 20 MHz.
Hence, a need remains in the art for an efficient, low cost system or method for amplifying multiple carrier signals while maintaining low intermodulation distortion levels.